1. Technical Field of the Invention
This invention relates to an improved perforation gun assembly and its unique phasing of the explosive shape charges to maximize production of oil and/or gas.
2. Description of Related Art
During the completion of a well, it is common to perforate the hydrocarbon containing formation with explosive charges to allow inflow of hydrocarbons to the wellbore. These charges are loaded in a perforation gun and are typically shaped charges that produce an explosive formed penetrating jet in a chosen direction. The effectiveness of the xe2x80x9cperforationxe2x80x9d is governed by many factors including, but not limited to, the orientation of the gun, the linear spacing and angular arrangement of the explosive charges, the properties of the formation, and the well casing geometry.
Conventional perforating guns come in two primary styles: hollow carrier guns and carrier strip guns. FIG. 1 illustrates a typical hollow carrier gun 10. A plurality of charges 12 are connected to each other by a detonation cord 14. The charges are spaced from each other by a strip 16. Common spacing for such guns is four to six charges 12 per foot. The explosive charges are protected from fluid in the well bore by a housing 18. The housing can be scalloped 20 to enhance charge performance and facilitate gun retrieval. FIG. 2 illustrates a capsule gun carrier strip assembly 24. Carrier strip assembly 24 includes first and second strip members 26 and 28. Strip members 26 and 28 are each elongated members having provisions for retaining shaped charges 30 thereto. Coupling the strip member 26 and 28 together, is a coupling plate 32. A detonation cord 25 connects each of the charges to a detonation energy source. In both figures, the charges are aligned in the same phase. This will produce a single phase of perforation in the formation as shown in FIG. 3.
Referring to FIG. 3, casing 2 is cemented in the well bore 6 by cement 4. During the creation of the well bore, the drilling process can damage a zone around the well. This zone can vary in diameter, but is generally shown by line 8. In this damaged zone, the permeabilty of the formation is particularly diminished. The perforation zone 9 should extend through the casing 2, the cement 4 and through the damaged zone 8 and into the formation 7. When all of the charges are aligned in the same phase, a single perforation phase 9 is formed. This is also called a zero phase perforation. It is well known that while a single phase allows fluid flow into the completed well, it is an incomplete solution. Fluid on the other side of the well from the zone 9 can have difficulty migrating through or around the damaged area and into the zone 9.
One solution is to stagger the shape charges at plus forty-five degrees and minus forty-five degrees from the original zone. FIG. 4 and FIG. 5 illustrate this configuration and the resulting xe2x80x9ctri-phasexe2x80x9d pattern it produces. The charges are evenly spaced along the carrier strip 52. A first charge 54 is provided with a positive forty-five degree offset. The second charge 56 is provided with no angular offset. The third charge 58 is provided with a minus forty-five degree offset. The fourth charge 60 is provided with no offset, and the final charge illustrated 62 is given a positive forty-five degree angular offset. The tri-phase gun 50 will produce three distinct phases of perforation tunnels 66, 68, and 70. Each tunnel should penetrate through the casing 2, cement 4, and the damage zone 8. A tri-phase pattern helps improve the formation""s inflow performance more than a zero phase pattern. Note that the gun 50 is located against one portion of the casing 2 rather than being suspended in the middle. This is true in the field because no well is perfectly vertical. When the gun is suspended into the well, gravity will naturally pull the gun against the low side of the casing.
When considering the phasing and order of angular offset with a tri-phase gun, one design consideration involves the effect of the detonation of the first charge with subsequent charges. In other words, when the first charge 54 detonates, the shock wave from that charge can physically damage or interrupt the second charge 56 as it detonates. The burn rate of the detonation cord 64 is particularly important. While cords burn at extremely fast rates, as the cord lengthens between charges, the more time will pass before the next charges detonates. For a given linear interval between charges, the cord between a charge located at plus forty-five degrees to a charge at zero offset is shorter than a cord between a charge at plus forty-five degrees and a charge at minus forty-five degrees. This is easily understood with reference to FIG. 4. Thus, it is preferable to minimize the cord length between adjacent charges. The order of detonation also is implicated. For example, the gun shown in FIG. 4 detonates charges in a sequence that maintains the shortest fuse length between charges: +45, 0, xe2x88x9245, 0, +45, 0 and so forth.
Another attempt at improving formation production involves the use of a six phase pattern also known as a sixty degree spiral phase pattern. A gun 80 is loaded with charges, each charge located a sixty degree offset from the previous charge. It will produce a spiral pattern similar to the one shown in FIG. 6. Unfortunately, one result that has been observed is that perhaps only three of the phases will perforate the formation all the way past the damage zone 8. As shown, perhaps only tunnels 82, 84, and 86 penetrate through the damage zone, while tunnels 88, 90, and 92 do not. This is caused by the fact that the gun will rest on the low side of the casing 2. The end result is that if the gun had six charges per foot, only three of the charges per foot had any meaningful impact on the formation. This results in a waste of explosive and a failure to achieve the optimum formation characteristics. Further, larger explosive charges may not be useable because of the limited outer diameter requirements of the gun.
A need exists for an improved method of and assembly for perforating a formation to achieve optimal inflow characteristics by producing novel and nonobvious phasing of the perforations. Such an assembly should minimize the risk of detonation interference from an adjacent charge. Such an assembly should also allow for the maximum number of charges per foot. Finally, the assembly should be able to produce the optimal results without any increase in the outer diameter of the assembly.
The present invention relates to an improved phasing of charges in a perforation gun as well as the improved gun that implements that phasing. The improvement overcomes the problems associated with multi-phase guns of the past that failed to account for the fact that the gun rested on the low side of the casing. In one embodiment of the present invention, the phasing is arranged so that there is a zero phase tunnel formed. The zero phase tunnel is located at approximately the location where the gun rests against the low side of the well. Further, tunnels are formed by shape charges at plus and minus forty-five degrees and at plus and minus ninety degrees. This can also be referred to as a penta-phase.
In another embodiment of the invention, charges can also be placed to allow for a plus and minus one hundred and thirty-five degrees pattern in addition to the penta-phase pattern described above. This expanded pattern can also be referred to as a hepta-phase pattern. By improving the phasing pattern of the perforation gun, valuable hydrocarbon fluids will encounter less resistance to flow into the well.
Another aspect of the present invention also relates to the order of detonation of the charges. The present invention minimizes the risk of interference from a previous detonation by minimizing the angular offset between adjacent charges. In other words, each charge in a sequence of charges is separated by a particular angular offset. The offset between adjacent charges is equal to a single multiple of that offset, rather than multiples of that offset. Thus, a penetration pattern is formed which oscillates between the two outer phase penetration tunnels.